Established pulmonary hypertension in rats was reversed by a combination of a HIF-2 alpha antagonist and a p53 agonist

Background and Purpose: Recent studies reported therapeutic effects of monotherapy with either tumour suppressor p53 (p53) agonist or hypoxia-inducible factor 2 alpha (HIF-2 alpha) antagonist for pulmonary hypertension (PH). This study investigated whether a combined treatment of p53 agonist, Nutlin3a, and HIF-2 alpha antagonist, PT2385, would be more effective than monotherapy, based on the cell type-divergent regulation of p53 in pulmonary arterial smooth muscle cells (PASMC) and endothelial cells (PAEC) in patients and animals with PH. Experimental Approach: The SU5416/hypoxia-induced PH (SuHx-PH) rat model was used, along with cultured human PASMC and PAEC. Western blot, RT-PCR, siRNA and immunohistochemical methods were used along with echocardiography and studies with isolated pulmonary arteries. Key Results: Hypoxia-induced proliferation of PASMC is associated with decreased p53, whereas hypoxia-induced PAEC apoptosis is associated with increased p53, via a HIF-2 alpha-dependent mechanism. Combined treatment with Nutlin3a and PT2385 is more effective by simultaneously inhibiting the hypoxia-induced PASMC proliferation and PAEC apoptosis, overcoming the side-effects of monotherapy. These are (i) Nutlin3a exacerbates hypoxia-induced PAEC apoptosis by inducing p53 in PAEC and (ii) PT2385 inhibits PAEC apoptosis because HIF-2 alpha is predominantly expressed in PAEC but lacks direct effects on the hypoxia-induced PASMC proliferation. In rats, combination treatment is more effective than monotherapy in reversing established SuHx-PH, especially in protecting pulmonary arterial vasculature, by normalizing smooth muscle thickening, protecting against endothelial damage and improving function. Conclusion and Implications: Combination treatment confers greater therapeutic efficacy against PH through a selective modulation of p53 and HIF-2 alpha in PASMC and PAEC.